1. Field of the Invention
The present invention relates generally to vascular grafts, and more particularly to a method and apparatus for forming the flanged polytetrafluoroethylene (PTFE) cuffed section from a tubular PTFE graft to form a flanged vascular graft for end-to-side anastomosis useful for purposes of bypassing an occluded or diseased section of a blood vessel or as an access graft for hemodialysis. The PTFE graft has an integral terminal PTFE flanged cuff section which permits an end-to-side anastomosis with a blood vessel in which the terminal PTFE flanged cuff section is sutured to the blood vessel and provides a PTFE-tissue interface between the graft and the blood vessel. The flanged PTFE graft is the subject of U.S. Ser. No. 09/125,907, which is hereby expressly incorporated by reference as being illustrative of types of flanged grafts useful for distal bypass or hemodialysis access grafts which may be made using the apparatus and method of the present invention.
2. Description of the Prior Art
The uses of cuff grafts for bypassing peripheral vascular occlusive conditions, particularly femoro-crural patch prostheses, or for hemodialysis access grafts are well known in the art. To date, however, either autologous grafts or synthetic grafts with a terminal cuff fashioned from venous tissue at the anastomotic site have been used. Examples of conventional cuffed grafts are the Miller collar described in J. H. Miller, The Use of the Vein Cuff and PTFE, in VASCULAR SURGICAL TECHNIQUES 2 ed., 276-286 (W. B. Saunders 1989), and the Taylor patch described in Taylor, R. S., et al, Improved technique for polytetrafluoroethylene bypass grafting: long-term results using anastomotic vein patches, Br. J. Surg., 79:348-354 (1992). Both the Miller graft and the Taylor graft are cuff grafts and each employs a PTFE graft with an autologous venous cuff at the anastomotic site. The Miller collar and the Taylor patch each use venous tissue at the anastomotic site to avoid a compliance mismatch at the PTFE-tissue interface.
The flanged PTFE graft in U.S. Ser. No. 09/125,907, hereby incorporated by reference, offers a new type of anastomosis for femoro-crural bypass or access grafting in which the graft is fabricated in a flared, double-bulb configuration. The inventive graft configuration offers an optimal geometry for the anastomosis as a function of hemodynamic properties. By optimizing blood flow from the bypass prosthesis to the artery, formation of intimal hyperplasia may be reduced with a concomitant increase in graft patency and decreased morbidity.
The apparatus of the present invention consists of an annular mold having a radially extending annular slot, forming an expansion port. The flanged cuff graft is made by first forming an unsintered tubular PTFE vascular graft by extruding a PTFE lubricant mixture into a billet to form a tubular extrudate, placing the extrudate in the annular mold, and forming an annular cuff by either: 1) application of a negative pressure to the expansion port, or 2) application of positive pressure, as by a highly compliant angioplasty balloon, through the tubular extrudate lumen, to radially displace a section of the tubular extrudate, thereby forming a cuffed graft.
Various different approaches have been taken to fabricate branched grafts. As early as 1938, U.S. Pat. No. 2,127,903 to Bowen disclosed a bioabsorbable surgically implantable graft made of animal tissue and a binder formed by wrapping strips of the treated animal tissue about a structural form. U.S. Pat. No. 4,909,979, issued Mar. 20, 1990 to Possis, discloses a method of shaping a human umbilical cord for use as a vascular graft. The method employs a mandrel to support and shape the umbilical cord during forming and curing of the cord. The forming and curing process provides a cord with a blood flow restrictor section. PTFE coatings are provided on the mandrel to facilitate mounting the umbilical cord onto the mandrel. A shaping section of the mandrel is provided with a plurality of vacuum openings in the mandrel. The umbilical cord is treated with ethanol and a vacuum applied until the cord is dehydrated. The cord is then exposed to a curative and fixative solution and a vacuum applied until the umbilical cord is cured substantially airtight and circumferentially compressed and compacted around the mandrel forming section.
U.S. Pat. No. 4,354,495, issued Oct.19, 1982 to Bodicky, discloses a method of connecting a PTFE tube to a hub made of a moldable plastic, e.g., polyurethane, acrylics, polyethylene, polycarbonates, etc. The method involves selectively heating a portion of the PTFE tube to form a bulge or protrusion, then inserting the bulge into a mold and molding the moldable plastic hub about the bulge in the mold. U.S. Pat. No. 4,957,508, issued Sep.18, 1990 to Kaneko et al., discloses an elastomeric medical tube having proximal and distal ends, outwardly flared. The outward flare of the ends is achieved by forming the inner and outer surfaces of the tube to exhibit inverse elastomeric properties, i.e., the inner surface exhibits a dilating force, while the outer surface exhibits a shrinking force. The tube is made of high molecular weight polymers, particularly, polyvinyl halide, polystyrene, polyolefin series polymers, polyester series condensates, cellulose series high polymers, polyurethane series high polymers, polysulfone series resins, polyamides, etc. along with copolymers or mixtures of these.
U.S. Pat. No. 5,387,236 to Noshiki et al., issued Feb. 7, 1995, discloses a vascular prosthesis and method of making a vascular prosthesis by providing a vascular prosthesis substrate made of PTFE or other microporous material, and depositing and capturing within the wall of the prosthesis substrate fragments of biological tissue. The biological tissue fragments may be vascular tissues, connective tissues, fat tissues and muscular tissues and/or vascular endothelial cells, smooth muscle cells and fibroblast cells. The impregnation process is conducted by depositing the cellular material on the inner wall of the graft and applying a pressure differential between the lumenal and ablumenal wall surfaces to drive the tissue fragments into the microporous matrix of the vascular prosthesis. U.S. Pat. No. 4,883,453 to Berry et al., issued Nov. 28, 1989, discloses an aorto-coronary bypass graft and a method of making the graft. The graft consists of a plate portion and at least one tube portion extending from the plate portion. The graft and plate are disclosed as being made of an electrostatically-spun fibrous structure. The graft is adhered to the plate by mounting the graft onto a mandrel, applying adhesive to the surface of the plate surrounding an opening in the plate, and passing the mandrel through an opening in the plate until the graft contacts the adhesive. The adhesive is any suitable adhesive for the materials forming the plate and the graft. According to the preferred embodiment described in this reference, the graft preferably has a tapered wall thickness, such that the graft wall thickness adjacent the plate is greater than that distant the plate.
U.S. Pat. No. 5,110,526 to Hayashi et al., issued May 5, 1992, discloses a process for producing molded PTFE articles. According to this process, unsintered PTFE extrudates are inserted into a sintering mold. The sintering mold has a diameter slightly larger than the outside diameter of the unsintered PTFE extrudate. Clearance between the outside diameter of the unsintered PTFE extrudate and the inside surface of the sintering mold is on the order of 2% of the diameter of the sintering mold. The extrudate is drawn into the sintering mold via a plug, inserted into the terminal lumen of the extrudate and a wire and take-up reel. The PTFE extrudate is cut to match the length of the sintering mold, and the sintering mold is sealed on the cut extrudate end. The assembly is transferred to a sintering oven and sintered. During sintering, the extrudate expands in contact with the sintering mold and conforms to the shape of the sintering mold. After cooling, the sintered extrudate contracts away from the sintering mold and assumes an even shape corresponding to the sintering mold.
U.S. Pat. No. 3,196,194 to Ely, Jr., et al., issued Jul. 20, 1965, discloses an extrusion process for making FEP-fluorocarbon tubing. The extrusion process consists of screw-extruding fluid FEP copolymer through a barrel extruder to form a tubular extrudate, placing the tubular extrudate into a heater, pressurizing the tubular extrudate to radially expand the FEP extrudate, and cooling the expanded extrudate to yield a heat shrinkable tube with memory function to the reduced diameter extrudate.
U.S. Pat. No. 4,503,568 to Madras, issued Mar. 12, 1985, discloses an arterial bypass prosthesis for end-to-side anastomosis and reduction of anastomotic hyperplasia. The arterial bypass prosthesis consists generally of a connector element including a tubular entrance member, a tubular exit member and a heel member. The tubular entrance receives and provides an entrance passage for blood flow. The tubular exit member is coupled to and angularly offset from the tubular entrance and provides a passage for the blood from the entrance member. The heel member extends substantially coaxially from the exit member. The distal end of the heel member is inserted through the open arteriotomy and into the portion of the vessel upstream of the arteriotomy. The heel may be solid or may include a passage continuous with the entrance and exist members. A throat portion is located intermediate the tubular entrance and exit members and a circumferential skirt substantially surrounds the throat portion. The skirt heals into the advential tissue of the blood vessel.
With particular reference to known methods for making PTFE materials, the following are cited as examples of the state and scope of the art. U.S. Pat. No. 4,482,516 to Bowman et al., issued Nov. 13, 1984, discloses a process for producing high strength expanded PTFE products having a coarse microstructure. The resulting PTFE microstructure is then defined by a "coarseness" index which purports to consider node size, i.e., height and width and fibril length. U.S. Pat. No. 5,376,110 to Tu et al, issued Dec. 27, 1994, discloses a method of making vascular grafts by collagen cross-linking conducted under the influence of alternating pressure across the graft wall. The alternating pressure aids in cross-linking the collagen fibers. U.S. Pat. No. 4,743,480 to Campbell et al., issued May 10, 1988, discloses a method for extruding and expanding tubular PTFE products in which a helical groove is machined into the extrusion barrel and/or the mandrel. Extrusion of a tubular PTFE product results in an extrudate having nodes angularly displaced between about 85-15 degrees from the longitudinal axis of the extrudate.
Finally, U.S. Pat. No. 4,234,535 to Okita, issued Nov. 18, 1980, discloses a process for forming expanded PTFE vascular grafts having fibers of smaller diameter at the inner surface of the tubing and fibers of at least two times diameter at the outer diameter of the tubing. The grafts are produced by a process in which PTFE tubular extrudates are formed, then placed onto drive and take-up capstans. The capstan drive system conveys the extrudate through a heater set at a temperature above 327.degree. C., then into a vacuum case which causes radial expansion of the extrudate at a temperature above 327.degree. C., then, after radial expansion, the vacuum case is cooled, by introduction of cooled air, to a temperature below sintering temperature, thereby fixing the tube at the expanded diameter and in the longitudinal direction by tension from the drive and take-up capstans. This patent also discloses and claims the use of cooling air conveyed through the tube lumen during the radial expansion process. By conveying cooled air through the tube lumen, the temperature at the lumenal surface is maintained below the PTFE sintering temperature. In this manner, differing fibril diameters at the lumenal and ablumenal surfaces are formed.
In current clinical practice, a peripheral anastomosis between a bypass or access prosthesis and a peripheral artery has been performed by either direct anastomosis, interposition of a venous segment at the anastomotic site, anastomosing the prosthesis with a long venous patch sutured into the artery (Linton Patch), enlargement of the prosthesis within the anastomotic region using a venous patch (Taylor Patch) or interposition of a venous cylinder between the prosthesis and the artery (Miller Collar). In femoro-distal grafting, there is growing evidence that compliance mismatch between the graft and the recipient artery and hemodynamic factors are a major cause of thrombosis and the development of subintimal hyperplasia at the anastornotic site.